Fast decoding of long codes

ABSTRACT

A method and apparatus for long code group determination. The long code group is determined based upon a number of symbols in a received codeword, wherein the number of symbols is less than the total number of symbols in the codeword. The long code group number can be determined either through the use of a table of code sequences or through a reduced complexity code search. The use of the of these techniques allows for a quick determination of the long code group used by a particular base station.

BACKGROUND

[0001] The present invention relates to digital radio systems, and morespecifically, to the determination of long code groups as part of theprocessing of a received signal in a spread spectrum radiocommunicationsystem.

[0002] Radiocommunication systems involve the transmission ofinformation over an air interface, for example by modulating a carrierfrequency with that information. Upon reception, a receiver attempts toaccurately extract the information from the received signal byperforming an appropriate demodulation technique. Some systems providechannelization using a spread spectrum technique known as code divisionmultiple access (CDMA). In some CDMA systems, the information datastream to be transmitted can first be coded or spread using a uniquespreading code and then combined with a long PN-sequence or a shorterscrambling-sequence, collectively referred to herein as “long codes”.The long codes can be planned from cell to cell so that neighboringcells use different long codes. The information data stream and the longcode can have the same or different bit rates. The multiplication of theinformation data stream with the unique spreading code and long coderesults in an output stream of chips.

[0003] To further understand the usage of long codes associated withsignal processing in a CDMA radiocommunication system, consider thefollowing example. FIG. 1 illustrates the use of base stations totransmit radio waves to mobile users (mobile stations) in a cellularsystem. In a CDMA system, base station 10 can transmit signals to mobilestations 14 and 15 as a single (composite) signal. The signal directedto mobile station 14 is typically coded with a short code that isorthogonal or mostly orthogonal to a short code that is used to code thesignal directed to mobile station 15. These signals are then scrambledwith a second code that is sometimes referred to as a long code,associated with base station 10. The sum of the two coded and spreadsignals is then transmitted by base station 10.

[0004] When mobile station 14 receives the composite signal, mobilestation 14 multiplies the spread signal with the long code and the shortcode to recreate the signal directed to mobile station 14 and the signaldirected to mobile station 15 is suppressed as interference noise.Similarly, mobile station 15 multiplies the spread signal with the longcode and the short code assigned to mobile station 15 to recreate thesignal directed to mobile station 15 and the signal directed to mobilestation 14 is suppressed as interference noise. To perform thisprocessing on the received signals mobile stations 14 and 15 must haveidentified the long code used to scramble the received signal, inaddition to learning or knowing the applicable short codes and havingattained time synchronization.

[0005] In an exemplary CDMA system, the mobile stations are able toidentify the long code used by a particular base station by listening toa control channel known as the synchronization channel (SCH). Decodingof the SCH is performed during both an initial “power-on”synchronization phase and when measurements of neighboring basestations' SCH are performed for cell reselection. One symbol istransmitted in the SCH during every slot. The SCH consists of twosubchannels, the Primary SCH (P-SCH) and the Secondary SCH (S-SCH),which are transmitted in parallel from the base station. The P-SCHalways carries the same symbol. A mobile station listens to the P-SCH todetect the timing of the symbols of the S-SCH, and thereby the slottiming. After the P-SCH is detected the S-SCH is read using the P-SCH asa phase reference.

[0006] However, when in initial synchronization a mobile station has noknowledge of which particular long code in the set of all available longcodes might be received. Moreover, during the initial synchronizationphase, frame synchronization to the S-SCH has not yet been achieved andtherefore, the mobile station has no knowledge when the first symbol ina code sequence is received. The decoder must therefore take intoaccount that all time shifts of each available long codeword can bereceived as part of its attempt to identify the long code needed toscramble a particular SCH's transmission.

[0007] As mentioned above, mobile stations 14 and 15 may be performingmeasurements on signals from other base stations in surrounding cells,i.e., base stations 20, 30 and 40, while communicating with base station10 to ensure that they are currently communicating with the best basestation(s). In order to perform the measurements it is necessary todetermine the long codes used by the various base stations. When mobilestations 14 and 15 are in the measurement mode they may receive a listof neighboring base stations and which long code group they are using.Hence, the search for specific codewords becomes limited, e.g.,identifying one codeword out of a group of sixteen.

[0008] It is desirable to perform the determination of the long codes asquickly as possible. For example, when an interfrequency handover isprepared, measurements of base stations on other frequencies must beperformed. Therefore, the time for each measurement should be minimizedbecause the normal traffic to the terminal is interrupted whenmeasurements are performed on other frequencies. Additionally, sincemobile stations typically use batteries as a source of power, it isdesirable to perform the decoding quickly, in order to use less power.Further, by performing the decoding quickly, the required amount ofhardware is minimized because the measurement hardware can be timeshared.

[0009] In conventional systems, a correlation for all 16 symbols areperformed for the 15 symbols over the frame. Then all long code groupcode sequences in any of the 15 phases are used for correlation with thereceived sequence and the sequence with the highest correlation value isselected as being the correct long code group. Performing thecorrelations according to this conventional method requires a complexdecoder because of the many operations which are required. For example,if there are 32 long code groups, then the number of sequences which areused as candidates are 15*32=480. Thus, the correlations for 480sequences must be calculated and the best one shall be selected. Anotherdrawback of this conventional method is that it requires a considerableamount of buffering to perform the required correlations. Further, ifthe number of long code groups are increased, the decoding complexity issubstantially increased. Accordingly, a method and apparatus forperforming long code group detection using less than the total number ofsymbols in the codewords would be desirable.

SUMMARY

[0010] The present invention relates to reducing the number of bits orsymbols which need to be evaluated in order to determine the particularlong code group associated with the particular long code used toscramble transmissions by a particular base station. According toexemplary embodiments of the present invention, a broadcast controlchannel includes a synchronization bit or symbol in each of a pluralityof time slots. The sequential nature of these bits or symbols can beused by the receiver to determine which long code group corresponds tothe received code sequence.

[0011] According to one exemplary embodiment of the present invention, atable of code sequences is built. The received symbols are compared tothe table and a determination is made as to the detected long codegroup. In an alternate exemplary embodiment of the present invention, aniterative process is used to determine a sequence of symbols whichidentify a particular long code group through the use of hard or softmetrics.

[0012] Since the present invention does not require the decoding andcorrelation of all of the symbols of a long code group to uniquelyidentify the long code group, the complexity of decoding is decreaseddue to the reduced amount memory required for buffering the receivedcode sequence. Further, the complexity of decoding is decreased since itis not necessary to determine the metric of all of the symbols in thereceived code sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will now be described with reference to theaccompanying drawings in which:

[0014]FIG. 1 illustrates a cellular radiocommunication system;

[0015]FIG. 2 depicts an exemplary physical and logical channel structurein which the present invention can be implemented;

[0016]FIG. 3 illustrates an exemplary method according to an exemplaryembodiment of the present invention;

[0017]FIG. 4 is a table which illustrates exemplary long code groupnumbers and the corresponding codewords;

[0018]FIG. 5 is a table which illustrates exemplary received codesequences and the corresponding detected long group code group number;

[0019]FIG. 6 is a flowchart illustrating an alternate method accordingto a second exemplary embodiment of the present invention;

[0020]FIG. 7 is table which illustrates received symbols andcorresponding metrics at different time periods; and

[0021]FIG. 8 is a table which illustrates candidate sequences andsurviving sequences.

DETAILED DESCRIPTION

[0022] In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular circuits,circuit components, techniques, etc. in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that the present invention may be practiced inother embodiments that depart from these specific details. In otherinstances, detailed descriptions of well-known methods, devices, andcircuits are omitted so as not to obscure the description of the presentinvention.

[0023] The exemplary radio communication systems discussed herein aredescribed as using a hybrid TDMA/CDMA methodology, in whichcommunication between the base station and the mobile terminals isperformed using spreading and scrambling codes, but wherein informationis also conveyed in discrete time slots. However, those skilled in theart will appreciate that the concepts disclosed herein may find use inother access methodologies. Likewise, some of the exemplary embodimentsprovide illustrative examples relating to an exemplary frame and logicalchannel structure which is under consideration for certain thirdgeneration radiocommunication systems, however, the techniques describedherein are equally applicable to equipment used in any system.

[0024] When the first frame of a desired information signal is receivedby a receiver, in most communication systems, the clock at thetransmitter and the clock at the receiver are not “locked”, i.e. theyare not synchronized in time. One part of this locking procedure iscommonly referred to as frame synchronization. For the purposes ofillustration, and not limitation, consider the exemplary frame andlogical channel format depicted in FIG. 2. Therefore, although aspecific number of radio frames are used in the description of anexemplary embodiment of the present invention, one of ordinary skill inthe art would be able to implement the present invention using adifferent number of radio frames per superframe. Therein seventy-tworadio frames of 10 ms each make up a superframe. Within each radio framethere are fifteen timeslots. Each time slot includes various types andfields of information, depending upon the channel to which itcorresponds. In FIG. 2, three such channels, primary common controlphysical channel (P-CCPCH), the common pilot channel (CPICH) and SCH,are depicted.

[0025] These three channels are broadcast control channels transmittedby the base station to all of the mobile stations in the area servicedby that base station, e.g., using different short codes known a prioriby the mobiles. As can be seen in the figure and as described above, theSCH includes one synchronization symbol (or bit) per timeslot. Thesynchronization symbol (or bit) in the S-SCH can be used fordetermination of the long code group as described below. CPICH carriespilot symbols which can be used by a receiver, for example, to performchannel estimation. The P-CCPCH is a logical information channel whichcan carry varying types of overhead information, e.g., identifying anetwork operator, sector, etc.

[0026] A first exemplary embodiment of the present invention will now bedescribed in conjunction with FIG. 3 and FIG. 4 which depict anexemplary method for identifying long code groups and a table of longcode groups and their corresponding codewords, respectively. Inaccordance with the purely illustrative embodiment, it is desirable todetermine the long code group using the minimum number of symbolsrequired to uniquely identify the codeword at step 310. In the exampleof FIG. 4, three symbols are necessary to uniquely detect the codeword.The minimum number of symbols required to uniquely identify a codewordcan be determined by a full search of the codewords. For example,initially all codewords could be searched to determine whether eachcodeword can be uniquely identified by two adjacent symbols. If it isdetermined that the codewords cannot be uniquely identified by twoadjacent symbols, then all the codewords are searched to determinewhether each codeword can be uniquely identified by three adjacentsymbols. This process is repeated and the number of symbols is increasedby one each iteration, until it is determined that each codeword can beuniquely identified. The process for determining the number of symbolswhich are required to uniquely identify a codeword is merely exemplary,and one skilled in the art will recognize that other methods can beimplemented for determining the minimum number of symbols for uniquelyidentifying a codeword.

[0027] In step 320, a table of all possible code sequences is builtbased upon the minimum number of symbols necessary to uniquely detectthe codeword, as determined in step 310. This table is illustrated inFIG. 5. The first column is a description of the detected code sequence,the second column shows which long code group corresponds to aparticular code sequence, and the third column contains comments forcode sequences which correspond to codewords from more than one longcode group. The codewords or bits are received from the S-SCH channel,in step 330. Then in step 340 the received codewords are compared to thecode sequences of FIG. 5. Based upon the comparison in step 340 the longcode group is determined in step 350.

[0028] If each long code group may include more than one long code anadditional step would be added to the method of FIG. 3. In this step,the mobile station would try to find the correct long code in the longcode group by testing all of the long codes to determine which long codeis the correct long code. Since the number of long codes in the longcode group is small and since the timing has been determined by thedetermination of the long code group, there is only a minimal amount ofprocessing required by the mobile station to determine which long codeout of the long code group is the correct long code.

[0029] It will be recognized that there are code sequences in the firstcolumn in the table of FIG. 5 which do not correspond to long code groupcodewords. These code sequences are provided to identify the long codegroup when there are reception errors in the codewords. For example, thesecond row of FIG. 5 contains the code sequence 1, 1, 2. However,referring to FIG. 4, there are no codewords which contain the codesequence 1, 1, 2. Accordingly, it is assumed that the first symbol 1 isa reception error. Using the two remaining symbols, it is determinedthat long code group 1 contains a code sequence of 1, 2, and hence, longcode group 1 would be selected.

[0030] Referring to FIG. 5, it will be recognized that due to receptionerrors, more than one long code group could be associated with areceived code sequence. When this occurs, the first long code group inthe table could be selected as being the long code group which isassociated with the received code sequence. Alternately, the long codegroup can be randomly selected from the group of possible long codegroups. Using either of these methods, a mobile station can attempt todecode the received traffic channel signals using the selected long codegroup, and if it is determined that the signals are not being correctlydecoded, the mobile station can attempt to decode the received trafficchannel signals using another of the remaining long code groups, untilthe proper long code group has been identified. Alternatively, insteadof attempting to decode the received traffic channel using another ofthe remaining long code groups, the mobile station can reattempt thedecoding of the incoming long codes, e.g., the mobile station canrestart the method illustrated in FIG. 3.

[0031] It can be seen that the method illustrated in FIG. 3 requiresvery large tables. Further, the length of the tables increases as theminimum number of symbols necessary to uniquely determine a long codegroup increases. In order to avoid the use of large tables, FIG. 6illustrates an alternative embodiment of the present invention for longcode group determination which involves a reduced complexity codesearch.

[0032] In the embodiment of FIG. 6 soft or hard metrics from the matchedfilters are used directly for decoding the received data. The followingmethod has several variables and for this exemplary embodiment thevariables are set such that K₁=3, Thr₁=2 and P=1.1. Kj represents thenumber of symbol sequences which survive elimination in stage i, thesesequences are herein referred to as surviving sequences. The value forK₁ could be a different value for each stage, e.g., to minimize thechance of throwing out a correct sequence K_(i) could be set to a largervalue for the first and second iterations of the method described below,i.e., for i=1 and i=2. Thr₁ is the metric for a decision at step i,which can change for each iteration, i.e., for each i. The value of theThr_(i) is selected based in part on the type of correlator used. P isthe ratio between the largest metric and the next largest metric and isselected so that the sequences are long enough to provide the correlatorwith a good selection of sequences. The description of FIG. 6 willinclude in a purely exemplary embodiment the long code groups of FIG. 4and will be described in conjunction with the tables of FIGS. 7 and 8.

[0033] The decoding begins at step 605. In step 610, the receivedwaveform is correlated to all N symbols, wherein N is equal to thenumber of symbols contained in the codeword alphabet. In the currentexemplary embodiment the alphabet contains the numbers 1-16, therefore,as illustrated in FIG. 7, the received waveform is correlated to all 16symbols. In step 615, i is set equal to one. In step 620, the K₁=3sequences with the largest correlation values are selected as survivalsequences. In this example the table in FIG. 7 shows that at time T₁ thethree symbols with the highest correlation values are 2, 3, and 4 havingmetrics of 0.5, 0.9 and 0.4, respectively. In step 625 it is determinedwhether the value of the largest correlation is larger than or equal toThr₁ and whether the largest correlation is P times larger than thesecond largest correlation. In the table of FIG. 8 it can be seen thatthe received symbol 3 has the highest metric, i.e., 0.9. Although 0.9 isgreater than P times the second largest metric, i.e., 0.9>1.1*0.5, themetric is less than the threshold of 2. Since the answer to one of thetwo questions in step 625 is no, i is incremented by 1 in step 630.

[0034] In step 635, the survival sequences from step 620 are used in thedetermination of the candidate sequences. In this example, the survivalsequences are the symbols 2, 3, and 4 which are compared to thecodewords of FIG. 4. A comparison reveals that each of the codewordscorresponding to the long code groups contain one 2, one 3, and one 4.Therefore in step 635, there will be three candidates from each of thefour codewords, for codeword 1 there will be candidates (2, 3), (3, 4),and (4, 5), for codeword 2 the candidates will be (2, 1), (3, 2), and(4, 3), for codeword 3 the candidates will be (2, 4), (3, 5), and (4,6), finally for codeword 4 the candidates will be (2, 15), (3, 1), and(4, 2).

[0035] In step 640 new correlations are made for the candidatesequences, which are illustrated in column T₂ of FIG. 7. Thecorrelations between received signal and candidate sequences of lengthi=2 are combined in step 645. This occurs by adding the metric of thefirst number of the sequence at time T₁ with the second number of thesequence at T₂. The result of this operation is shown in the secondcolumn of FIG. 8. The process then returns to step 620 and the K₂=3surviving sequences from FIG. 8, i.e., those with the largest metrics,are selected. In the present example the sequences chosen are (3, 4),(3, 5), and (3, 1) with metrics of 1.3, 1.4, and 1.2, respectively.Although the sequence (3, 2), like the sequence (3, 1), has a metric of1.2, it is not chosen since only the three largest metrics are chosen.However, this decision is arbitrary and the sequence (3, 2) could bechosen over the sequence (3, 1). Further, one skilled in the art willrecognize that when two sequences have the same metric both sequencescan be selected. However, this would result in an increased decodingcomplexity, due to the increased number of sequences which are requiredto be decoded in the steps described below.

[0036] In step 625 it is determined that the metric 1.4 is less thanThr₂. Therefore, i is incremented by one in step 630, and the candidatesequences of length 3 are chosen in step 635. A comparison of thesurviving sequence above with the codewords of FIG. 4 indicates thatthere are only three candidate sequences produced from the survivingsequences. New correlations for time T₃ are calculated and thecorrelations are combined with the candidate sequences in steps 640 and645. FIG. 8 shows that the three candidate sequences at time T₃ are (3,4, 5), (3, 5, 7), and (3, 1, 16) with metrics of 2.0, 1.8, and 1.5,respectively. These sequences are selected as survival sequences in step620.

[0037] In step 625 it is determined that candidate sequence (3, 4, 5)has the necessary metric greater than the threshold of 2.0 and isgreater than 1.1 times the second largest correlation, i.e., (3, 5, 7).Then in step 650 it is determined whether the code sequence is longenough to uniquely detect a code word. The determination of whether thecode sequence is long enough to uniquely detect a code word can beperformed as described above with regard to FIG. 3, or by using anyother known method. If it is determined that the code sequence is notlong enough to be uniquely detected then the length of the metrics isincremented by one in step 630 and proceeds as described above. If it isdetermined in step 650 that the code sequence is long enough to beuniquely detected then a decision can be made as to the frame boundaryand the transmitted codeword. Hence, candidate sequence (3, 4, 5)indicates that long code group 1 should be chosen.

[0038] Although it has been mentioned that the result attained in step655 can be used for the determination of a frame boundary, a descriptionof how to implement such a determination is not particularly relevant tothis description. However, the interested reader is referred to U.S.patent application No. 09/100,233 “Frame Synchronization Techniques andSystems for Spread Spectrum Radiocommunication” filed Jun. 19, 1998, thedisclosure of which is incorporated here by reference.

[0039] The present invention could be advantageously used in a CDMAsystem which operates across at least two frequency bands. For example,referring to FIG. 1, if base stations 10, 20, 30 and 40 each operated ona different frequency band, in order for a mobile station to makemeasurements on surrounding base stations, the mobile station would haveto switch frequencies. Since a mobile station may be in communicationwith base station 10, the mobile station would require a second receiverto simultaneously receive signals over two different frequency bands.However, since according to the present invention a mobile station onlyneeds to receive part of the synchronizing sequence, the mobile stationcan use the remaining time for which it would normally receive theremainder of the synchronizing sequence, for measuring on differentfrequency bands.

[0040] The present invention has been described by way of exemplaryembodiments to which the invention is not limited. Modifications andchanges will occur to those skilled in the art without departing fromthe spirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A method for identifying a long code groupcomprising the steps of: determining a number of symbols necessary foruniquely detecting a long code group; building a table of all codesequences with a length equal to the number of symbols necessary foruniquely detecting a long code group; receiving a number of codewords,wherein said number of codewords are equal to said number of symbolsnecessary for uniquely detecting a long code group; comparing saidnumber of codewords with the table of all code sequences; andidentifying said long code group base upon said comparison.
 2. Themethod of claim 1, wherein the step of determining a number of symbolsnecessary for uniquely detecting a long code group comprises the stepsof: searching all codewords to determine whether each codeword can beuniquely identified by a first number of adjacent symbols; and if eachcodeword cannot be uniquely identified by the first number of adjacentsymbols then increasing the first number of adjacent symbols by onesymbol and repeating the searching step.
 3. The method of claim 1,wherein the codewords are received from a control channel.
 4. The methodof claim 1, wherein if more than one code sequence matches the number ofreceived codewords, then the comparing step comprises the step of:attempting to decode received traffic channel signals using a long codegroup associated with each code sequence.
 5. A method for decoding acodeword corresponding to a long code group, wherein a waveform isreceived and a first number of symbols are correlated with said receivedwaveform to determine metrics corresponding to each of said number ofsymbols, further comprising the steps of: selecting a first number ofsequences with the largest metrics; comparing a metric of the sequencewith the largest metric of said first number of sequences with athreshold; comparing the metric of the sequence with the largest metricwith a metric of the sequence with the second largest metric of saidfirst number of sequences; determining whether said sequence with thelargest metric of said first number of sequences is long enough touniquely detect a codeword; and selecting said sequence with the largestmetric as the codeword corresponding to said long code group if metricof the sequence with the largest metric of said first number ofsequences is greater than said threshold, a certain factor greater thansaid metric of the second largest metric of said first number ofsequences, and the sequence with the largest metric of the first numberof sequences is long enough to uniquely detect.
 6. The method of claim 5further comprising the step of: selecting a second number of sequenceswith the largest metrics as candidate sequences if the largest metric ofsaid first number of sequences is: less than said threshold, or not acertain factor greater than said second largest metric of said firstnumber of sequences, or not long enough to uniquely detect.
 7. Themethod of claim 6, wherein said step of selecting said second number ofsequences further comprises the steps of: determining a number of saidcandidate sequences using said first number of sequences with thelargest metrics; correlating a second number of symbols with saidreceived waveform to determine metrics corresponding to each of saidsecond number of symbols; calculating a total metric of each of saidnumber of candidate sequences by combining metrics of said first numberof symbols and metrics of said second number of symbols which correspondto each candidate sequence.
 8. The method of claim 7, further comprisingthe steps of: repeating the steps of claim 2 using said candidatesequences in place of said first number of sequences.
 9. The method ofclaim 7, wherein the waveform is received in a control channel.
 10. Anapparatus for determining a long code group comprising: means fordetermining a number of symbols necessary for uniquely detecting a longcode group; means for building a table of all code sequences with alength equal to said number of symbols necessary for uniquely detectinga long code group; means for receiving a number of codewords, whereinsaid number of codewords are equal to said number of symbols necessaryfor uniquely detecting a long code group; means for comparing saidnumber of codewords with the table of all code sequences; and means fordetermining said long code group base upon said comparison.
 11. Theapparatus of claim 10, wherein the means for determining a number ofsymbols necessary for uniquely detecting a long code group comprises:means for searching all codewords to determine whether each codeword canbe uniquely identified by a first number of adjacent symbols; and meansfor increasing the first number of adjacent symbols by one symbol andrepeating the searching step, if each codeword cannot be uniquelyidentified by the first number of adjacent symbols.
 12. The apparatus ofclaim 10, wherein the codewords are received from a control channel. 13.The apparatus of claim 10, wherein the apparatus is a mobile station.14. An apparatus for decoding a codeword corresponding to a long codegroup, wherein a waveform is received and a first number of symbols arecorrelated with said received waveform to determine metricscorresponding to each of said number of symbols, further comprising:means for selecting a first number of sequences with the largestmetrics; means for comparing a metric of the sequence with the largestmetric of said first number of sequences with a threshold; means forcomparing the metric of the sequence with the largest metric with ametric of the sequence with the second largest metric of said firstnumber of sequences; means for determining whether said sequence withthe largest metric of said first number of sequences is long enough touniquely detect a codeword; and means for selecting said sequence withthe largest metric as the codeword corresponding to said long code groupif metric of the sequence with the largest metric of said first numberof sequences is greater than said threshold, a certain factor greaterthan said metric of the second largest metric of said first number ofsequences, and the sequence with the largest metric of the first numberof sequences is long enough to uniquely detect.
 15. The apparatus ofclaim 14 further comprising: means for selecting a second number ofsequences with the largest metrics as candidate sequences if the largestmetric of said first number of sequences is: less than said threshold,or not a certain factor greater than said second largest metric of saidfirst number of sequences, or not long enough to uniquely detect. 16.The apparatus of claim 15, wherein said means for selecting said secondnumber of sequences further comprises: means for determining a number ofsaid candidate sequences using said first number of sequences with thelargest metrics; means for correlating a second number of symbols withsaid received waveform to determine metrics corresponding to each ofsaid second number of symbols; means for calculating a total metric ofeach of said number of candidate sequences by combining metrics of saidfirst number of symbols and metrics of said second number of symbolswhich correspond to each candidate sequence.
 17. The apparatus of claim16, wherein the apparatus is a mobile station.
 18. The apparatus ofclaim 14, wherein the waveform is received from a control channel.